Most broadband OFDM (Orthogonal Frequency Division Multiplexing) systems comprise a number of modulated data subcarriers, which are transmitted over large frequency bandwidths. The advantage of OFDM is that it improves the data throughput by lowering the data rates on each subcarrier but increasing the data throughput by simply transmitting more subcarriers. Decreasing the data rate on each sub-carrier when noise or channel problems exist allows the increased data transfer to exist. These techniques have proven very effective in many wireless and fixed wire systems such as xDSL, IEEE802.11, IEEE802.16 and LTE systems. High data rates are therefore possible. Additional methods of reducing inter-symbol interference and multi-channel fading are also incorporated through including a Guard Interval or Cyclic prefix extension or inserting pilot tones on fixed location subcarriers within the N subcarrier frame transmission. Although OFDM provides many advantages, two problems associated with such systems are high instantaneous values of PAPR (Peak to Average Power Ratios) and fast fading multipath mobile channel.
Due to the summation of the individual subcarriers at a transmitter output, there is a probability that very high instantaneous values of power will exist. Unfortunately the dynamic range of the transmitter high power amplifiers (HPA) is usually lower than the dynamic range of the PAPR variations thus severely distorting the signals. In addition, as most HPAs are non-linear in nature distortion of the high PAPR signals always exists. The consequences of high PAPR values is the introduction of bit errors at the receiver as well as potential inter-channel or inter-frequency interference due to frequency spectral splatter arising from the HPA distortion. Numerous solutions to reducing PAPR in OFDM systems exist and these include, clipping, filtering, phase shifting, block coding, selected mapping, and tone insertion techniques. However, these have their own disadvantages, which include, introduction of distortion and particularly the requirement of transmitting side-information or extra information. This extra information is required to allow the receiver to know how the data or pilots have been modified to reduce the PAPR causing a reduction in throughout and also influencing the PAPR itself.
Mobile OFDM systems suffer from the presence of multi-path channel fading. As the fading becomes severe, particularly at high mobile velocities or high GHz frequency transmission carriers, multi-path fading becomes more difficult to eliminate causing a dramatic decrease in Quality of Service as the bit error rate (BER) increases and the data rate reduces. The solution to helping eliminate multipath fading is the insertion of pilot tones or training sequences within OFDM systems. These pilot tones allow receiver equalisers to evaluate the channel conditions and to correct the data. The main problem with pilot tones is that they are only of limited ability in correcting the data. The channel conditions can vary significantly from subcarrier to subcarrier and from frame to frame in rapidly changing fading environments thus not limiting the lower bound on BER.
WO2004/084513, the contents of which are incorporated herein by reference, describes the concept of embedding different forms of pilots into OFDM transmissions. In this, a control data block is embedded within a plurality of real data blocks. The real data blocks are modulated with one or more transmission sub-carrier signals and the control data block is modulated with every transmission sub-carrier used to modulate the real data blocks. The techniques described in WO2004/084513 allow a received signal to be decoded without knowledge of the control phase angle.
Although WO2004/084513 addresses the issue of dynamic variability between time frames, it does not address the problem of dynamic variability between individual subcarriers within an OFDM frame.